I was checking out the latest pics in from Opportunity and noticed this link on the MarsRovers website:

Scientists Need Your Help!

Mars scientists are asking students from around the world to help them understand the red planet. Send in a rock collected by you or your classroom from your region of the world, and we will use a special tool like the one on the rover to tell you what it's made of. Then everyone can compare their rocks to the ones found on Mars.

We'll post a picture of your rock on the web, and give you a report on what kind of rock it is. We'll also send you an official certificate and Mars sticker for your contribution.
See link for more details.

So send 'em your rocks, kids!